Trifocal lens.



H. BOLDE.

TRIFOGAL LENS.

APPLICATION FILED NOV.15, 1912.

1,082,491. Patented Dec. 30, 1913.

UNITED STATES PATENT omriou.

HENRY 30mm, on NEW YORK, N. Y., ASSIGNQR T0 rm: nnrnowrrz MANUFACTURING60., A coRPoRArIo or, NEW JERSEY.

rnirooAn LENS.

Specification of Letters Patent. Application filed Novembe'rlfi, 191aSerial.No.-731,492.

Patented Dec. 30, 1913.

To all whom it may concern:

- Be it known that I, HENRY BOLDE, a citizen of the United States,residing at the city of New York, in the borough of Brooklyn and Stateof New York, have invented certain new and useful Improvements inTrifocal Lenses, of which the following is a full, clear,'and exactdescription.

This invention relates to tri-focal lenses for eyeglasses and spectaclesand has special reference to that class of multifocal lenses in which acompleted lens consists of a majorlens provided with one or, morecavities in which is or are seated a minor lens or lenses of glassmaterial of different refractive index, the surface containing the minorlenses being of even curvature throughout.

The primary object of the invention is to. provide a construction oftri-focal lens in the grinding of whose surfaces the relative areas ofthe fields of vision of the two minor lenses will be under control.

My understanding -is that heretofore in the construction of trifocallenses the twominor lenses have both been seated on, or

applied to, one of the faces of the major lens, so that when such lensesare seated in,

cavities of the major lens in the process of making, and the entire faceof such lens is then ground to an even curvature, the circumscribedexposed area of the minor lenses must necessarily be altered in unequalratio due to the fact that the curvatures of the inner faces of theminor lenses are different and are ground away with unequal rapidity.

My invention therefore consists of a trifocal lens comprising a majorlens and twominor lenses, the latter being respectively seated in orapplied to the two faces ofthe major lens, whereby the grinding of thetwo composite faces of the lens can be accomplished independently ofeach other and the is a section on line 4, 4 of Fig. 3, and Fig. 5

is a diagram illustratin the lack of control of the relative areas 0 thefields of VlSlOIl eient of refraction.

in a composite lens when both of the minor i'enses are applied to oneface of the major ens.

In the various figures, a indicates the major lens and band a therespective minor lenses of a trifocal lens. The minor lenses are seatedin cavities in the surfaces of the major lens, one being located in oneface while the other is located in the other face of the major lens. Theminor lenses are attached to the major lens preferably by fus ing sothat they become integral therewith, although this is not essential asthey may be cemented in place by means of. Canada balsam or othertransparentmaterial.

The lenses band 0 are necessarily made of glass material having adifferent refraca is made, as otherwise no difference in focal powerwould be obtained when the exterior surface of the completed lens isground to an even curvature throughout, as is the case with respect tomy invention. For instance if lens a is made of crown glass, the lenses6 and 0 might be made of flint glass which has a different coefii- Asbetween the two lenses 6 and 0 however, which may be of material havingthe same co-eflicient of refraction, the different focal powers isobtained by giving to their inner surfaces, which rest against the-majorlens, different curvatures. Thus in the finished lens we have threefields of vision each of different focal power from the other two. Oneof these fields is fur nished by that portion of the major lens notoverlayed by either of the minor lenses as for instance all of thatportion outside of the edge of lensb in Fig. 1 or that portion outsideofthe two circles b and c in Fig. 3. The second field in the structureof Fig. 1 is the annulus which lies between the boundaries of the twolenses b and 0, that portion located above the lens a being most used,while in Fig. 3 the second field is represented by the area of lens b.The third field is represented in both construc-' tions by the area oflens 0. It is desirable to.make the area of these different fields ofvision conform to the requirements of use; thus for distance it shouldbe possible to shift the line of vision to the right and loft, hence themajor lens a which has the largest field should be" used for thispurpose. For reading or near vision where the eye does not wander, thesmallest field may be used, hence c is adapted for this purpose. Formiddle distance, where a person wlshes to see where he is stepping forinstance, the eye requires a little more latitude than in reading andthe annulus of lens I) in Fig. 1 is adapted for this purpose. For middledistance however the field may be of practically the same size as thereading field, as shown in Fig. 3. The various fields of vision must inany case have a certain minimum area. So that with these things in viewit is obviously desirable to devise a lens in the construction of whichthe areas of the various fields can be controlled.

In constructing tri-focal lenses of the kind herein referred to, it iscustomary to grindand polish the cavities or countersinks for the minorlenses in the rough blank of the major lens; then to secure rough blanksof the minor lenses, in said countersinks, the minor lens blanks havingfirst been prepared with a polished surface to fit the countersinkswhich they are to occupy and finally the composite blank is ground to aneven curvature throughout, the grinding tool cutting to a uniform depthinto the major and minor lenses. Now, if the two minor lenses arelocated on the same side of the major lens, it is obvious that, sincethey are of different curvatures on their inner faces, the area of theirouter surfaces will be unequally reduced by the grinding tool. Forinstance, if it be assumed that the grinding has reached the stageindicated by the line e in Fig. 5, and the diameter of each of the minorfields b and c is then equal as shown by the distances between 7 and. g,and h and 2', and it is found that further grinding is ture, in thepresence of necessary, say to the depth indicated b the line 6, then itwill be found that the iam eter f and g is considerably less than thediameter ht". Thisis due to the fact that the radius of the curvature pis greater than that of q. Consequently no control can be exercised overthe relative diameters. or areas of the two minor fields of vision. Ifhowever, one of the minor lenses is located on each side of the majorlens as shown in Figs. 1 to 4, it is obvious that thegrinding can becontinued on one side until the desired diameter of the one minor fieldis obtained, and then the grinding ofith'e other side can be carried onuntil the desired diameter of the minor field thereon -is obtained,without in the meantime disturbing the diameter of the fieldalreadydetermined on the first side.

Among the advantages which spring from I the ability to control thesediameters of the fields, is the fact that fewer lenses are spoiled inthe grinding.

Having described my claim 1. A tri-focal lens comprising a majorinvention, I

lens and two minor lenses seated respectively in the two faces thereof.

2. A tri-focal lens comprising a ma or lens and two minor lenses seatedrespectively in the two faces thereof, both faces of the completed lensbeing of uniform curvature throughout.

In witness whereof, I subscribe my signatwo witnesses. HENRY BOLDE.Witnesses:

AMELIA C. MoEsoHnN, FREDERICK L. Hreenvs.

